Being a half gallon short shouldn’t make a huge difference as dry hopping is more of an art than a science anyway which is why they gave the 1-2 week range in the first place. In my experience most of the flavour and aroma from the hops passes to the beer by the end of the first week. One thing you could do is draw off a little of the beer and smell/taste it to see if you are satisfied, I’ve even added more dry hops later when I didn’t feel the first addition was enough. Remember as you have already found that adding carbonation will change both the aroma and flavour characteristics more.

In the future if you want to stay more true to the recipe you should adjust the hop addition to match the volume rather than trying to adjust the time, but like I said it shouldn't make a big difference at that volume.

Adrian, you can make pretty much any style at home. However, once you get to ~10% ABV, you have some additional challenges because most yeasts can't handle it. So, then you have to figure out whether to pitch a second fermentation with champagne yeast (or something) or just live with the amount of residual sugar.

The IPA I have in the fermenter now measured ~8% when I transferred it to secondary on Oct. 23. It can only go up the longer I leave it in secondary, but the yeast do most of their work eating sugars, pissing out alcohol, and farting CO2 during the first week, so I don't anticipate it going up by any more than a couple tenths from there.

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"I wish I had documented more…" said nobody on their death bed, ever.

"Well, ya see, Norm, it's like this. A herd of buffalo can only move as fast as the slowest buffalo. And when the herd is hunted, it is the slowest and weakest ones at the back that are killed first. This natural selection is good for the herd as a whole, because the general speed and health of the whole group keeps improving by the regular killing of the weakest members! ; In much the same way, the human brain can only operate as fast as the slowest brain cells. Excessive intake of alcohol, as we know, kills brain cells. But naturally, it attacks the slowest and weakest brain cells first.. In this way, regular consumption of beer eliminates the weaker brain cells, making the brain a faster and more efficient machine! That's why you always feel smarter after a few beers."